1. Field of the Invention
The present invention relates to an optical recording and reproducing method and a apparatus for the same and, more particularly, it relates to a multi-value information recording and reproducing method and an apparatus for the same intended to increase its recording density.
2. Description of the Related Art
CDs and laser disks of the read-only type (ROM), Te-metal thin films and organic pigment films of the write-only type (WORM), and magnetooptic films and Calkogenite phase change films of the reversible type (E-DRAW) are used as the conventional recording media employed by this kind of the method and apparatus. In a case where a certain information is to be recorded on one of these recording media, recording laser beam converged to a wavelength is shot onto the recording medium and run thereon in the track length direction thereof to form plural marks thereon at a certain interval. Information which has been coded corresponding to the interval between these marks is recorded on the recording medium. In a case where the certain information is to be reproduced from the recording medium, the recording medium is scanned in the track length direction thereof to detect diffraction beam which changes depending upon whether or not the marks are present on the track of the recording medium.
The following various methods have been proposed these days to increase the recording density: Methods of changing the reflection factor of each of the marks step by step. of changing the depth of the mark, of changing the size of the mark, changing the interval between the marks, and of making small the interval between the tracks. In addition, there is a method of recording information on each of plural layer films which form the recording medium to increase the recording density. These methods mentioned above are intended to straight increase pieces of information recorded by changing a mark step by step.
Japanese Patent Disclosure Sho 61-129748 discloses a further method, whereby a set of information is converted to a pattern (or optical interference or diffraction pattern) at a certain area different in surface contour and recorded on the recording medium and whereby the optical interference or diffraction pattern created by the converted pattern is obtained as the set of information at the time of reproduction.
In the case of these multi-value information recording methods, however, it is difficult to adjust the state of mark recording step by step to accurately detect signals applied from the marks and the extent to which information can be multi-valued is limited to 2 to 4 values. Further, the method of recording a set of information at a certain area as a pattern is used by the simple systems which carry out neither tracking nor focusing, but the recording density achieved by the method is low. Therefore, this method cannot be applied to those systems which are intended to achieve high recording density. Further, it is supposed that the recording medium is made large in size to make pieces of information recorded more in value, but when pieces of information are made more and more in value, the whole of the system must be made large in size.